It appears to be possible in theory with the current 2012 Republican Presidential Nominee market. The current bids sum to quite a bit more than 100 so by selling contracts on every outcome you should receive more than 100 and you will never have to pay out more than 100 so you should have a guaranteed profit.
The problem is that the rules for this market say
This market is not linked to allow cross-margining of positions. You will be margined individually on each position, long or short.
which suggests to me that you would be required to put up margin to cover the possibility of losing every bet which is clearly not a possible outcome. That makes it impractical to take advantage of the arbitrage opportunity.
Liquidity might also be an issue in this case - you might not find buyers for all your contracts at the quoted market price.
You're correct about Intrade's requirement to front the money to cover your position in all cases until the contract ends or your sell it.
However:
The current bids sum to quite a bit more than 100 so by selling contracts on every outcome you should receive more than 100 and you will never have to pay out more than 100 so you should have a guaranteed profit.
That doesn't follow. Even if the bids sum to more than a hundred, you have put up the other fraction of $10 for all of those n contracts. With a lot of the bids very low, then in order to cover all ...
I've noticed something very curious on Intrade markets for 2012 Republican Presidential Nominee - Ron Paul gets 3.5% of getting a nomination - a value that's clearly (and spare me EMH here) wishful thinking of Ron Paul supporters more than any genuine estimate.
And this brings me to a question - if prediction markets overestimate chance of winning of some rare case, how can I profit from that? Naively if I know true chance is 1%, I win $3.5 99% of the time, and lose $97.5 1% of the time, for expected payoff of $2.5. But my maximum loss is 39x higher than my expected profit, and I won't be getting any money out of it for three more years.
I'd need to bet significant amount to earn any money out of it, and that would require accepting 39x as high maximum loss. No reasonably prediction market would accept this kind of leverage without some collateral, nor could I get any reasonable loan for it at rates that would make this arbitrage profitable.
The only way I can think of would be convincing someone with plenty of money that I'm right, and have him provide me with collateral for some (probably very high) portion of the payoff. But if results depend on my ability to convince rich people, that's not prediction market! None of this is a problem for people trying to artificially pump estimates for Ron Paul - they'll just take the loss, and write it off as marketing expense.
None of these problems occur if some position is vastly overestimated, like 60% estimate if I know true value to be 40% - this would be a cheap bet - maximum loss of $40 for expected profit of $20, and people who want to pump it need to take about as much risk as people who want to bring it back to the true value, not a lot more.
I'm confused. Is there some nice way to arbitrage this, or is this an inherent weakness of prediction markets and we should only trust positions they pick as leaders, not chances of their long tail?